1. Field of the Invention
The present invention relates to a servo device such as a tracking servo device in a player for playing a recording medium including a disk and so on.
2. Description of the Related Background Art
In an optical disk player, a tracking servo device is indispensable for controlling a reading point of a pickup to the center of a track on an optical disk. The tracking servo device generates a tracking error signal indicative of the amount of deviation of the reading point with respect to the track in the radial direction of the disk, such that the pickup operates in response to the tracking error signal. In addition, when blemishes are on a track of an optical disk, it is impossible to generate an appropriate tracking error signal. Thus, some tracking servo devices are provided with a tracking hold function as a servo hold function, for solving the problem.
In a conventional tracking servo device having a tracking hold function, as illustrated in FIG. 1, a tracking error signal generator 2 generates a tracking error signal indicative of the amount of deviation of a reading point with respect to a track in the radial direction of a disk based on an output signal of a pickup 1. For generating a tracking error signal, three methods are known, a three-beam method, a push-pull method, and a time difference detecting method. The tracking error signal from the tracking error signal generator 2 is supplied to a tracking driving circuit 5 as a tracking driving signal through an equalizer 3 and a change-over switch 4. The tracking driving circuit 5 drives a tracking actuator (not shown) within the pickup 1 in response to the tracking driving signal.
For the tracking hold function, an LPF (low pass filter) 7 is disposed between the tracking error signal generator 2 and the change-over switch 4. The LPF 7 outputs a low frequency component (a DC offset component due to eccentricity of the optical disk, or the like) other than a rapid change component in the tracking error signal. A defect detector 8 is also disposed for detecting blemishes (including defects) at a reading point based on an output signal of the pickup 1. The defect detector 8 generates a DEF signal when it detects a blemish, so that the change-over switch 4, responsive to the DEF signal, supplies the tracking driving circuit 5 with an output signal of the LPF 7 as a servo hold signal in place of an output signal of the equalizer 3.
With the configuration described above, when an RF signal, i.e., a read signal output from the pickup 1, has decreased to a lower level due to a blemish on the disk as illustrated in FIG. 2A, the tracking error signal from the tracking error signal generator 2 fluctuates up and down and therefore becomes unstable, as illustrated in FIG. 2B. The defect detector 8 generates the DEF signal of a high level as illustrated in FIG. 2C due to the decrease of the RF signal level. As described above, the change-over switch 4 is responsive to the DEF signal to supply the tracking driving circuit 5 with the output signal of the LPF 7 in place of the output signal of the equalizer 3. The output signal of the LPF 7, the level of which is substantially constant as illustrated in FIG. 2D during the time that the DEF signal is generated, is supplied to the tracking driving circuit 5 as the tracking driving signal.
In the conventional tracking servo device provided with a tracking hold function as described, the tracking error signal supplied to the tracking driving circuit 5 immediately before the generation of the DEF signal may include a large fluctuating component as indicated by a circle X in FIG. 2E. The large fluctuating component is caused by a detection sensitivity of the defect detector 8 which is not set so high. If the defect detector 8 has a higher detection sensitivity, the large fluctuating component is improved. Since the defect detector 8 with the higher detection sensitivity detects even small blemishes which do not require the tracking hold function to operate and then generates the DEF signal, the detection sensitivity of the defect detector 8 should not be set at a high level.
However, since the tracking actuator has an operation characteristic of an LPF and performs an integration operation, a large fluctuating component included in the tracking error signal supplied to the tracking driving circuit 5 immediately before the generation of the DEF signal causes the tracking actuator to move the reading point gradually away from the center of the track during the generation of the DEF signal, as illustrated in FIG. 2F, thereby preventing the tracking hold function from sufficiently taking effect.
The problem is not limited to the tracking servo device but similarly occurs in other servo devices having the servo hold function, such as a focus servo device.
It is therefore an object of the present invention to provide a servo device for a recording medium player which is capable of maintaining an appropriate servo state even if a reading point has passed through a portion including a blemish on a recording medium.
The present invention provides a servo device for a recording medium player, which generates a servo error signal indicative of an amount of deviation of a position at which a pickup of the recording medium player is reading for playing a recording medium, generates a servo driving signal from the servo error signal through an equalizer, and controls a reading operation of the pickup in accordance with the servo driving signal. The servo device includes blemish detecting means for detecting a blemish at a reading point by the pickup on the recording medium to generate a blemish detecting signal when a blemish is detected; a first low frequency extracting means for extracting a low frequency component from the servo error signal; and control means for generating a servo hold signal based on an output signal of the first low frequency extracting means, wherein the servo hold signal includes, immediately after the generation of the blemish detecting signal, a signal of opposite polarity to the polarity of the servo error signal immediately before generation of the blemish detecting signal, and for controlling the reading operation of the pickup in accordance with the servo hold signal in place of the servo driving signal during the time that the blemish detecting signal is generated.
According to the servo device for a recording medium player of the present invention, when a servo error signal includes a large fluctuating component immediately before the generation of a blemish detecting signal, a servo hold signal is generated including a signal of opposite polarity to that of the servo error signal immediately after the generation of the blemish detecting signal, so that a reading operation of a pickup is controlled in accordance with the servo hold signal in place of a servo driving signal during the time that the blemish detecting signal is generated. Therefore, even if a reading point passes through a blemish on a recording medium, it is possible to maintain an appropriate servo state immediately before passing through the blemish.